1. Field of the Invention
The present invention relates to a portable telephone equipment, and, more particularly, to a resuming function to return the portable telephone equipment which is interrupted by a short break in power supply to the state before the short break.
2. Description of the Prior Art
Some portable telephones of the prior art resume their pre-break states when a short break in power supply occurs if the period of the break is shorter than the discharge time of a capacitor incorporated in the hardware. This is because the capacitor which is charged during normal operation continues to discharge according to the time constant of a circuit during a short break caused by battery exchange or the like. When the equipment recovers from a short break, if the terminal voltage of this capacitor is still higher than the threshold value, a power failure is determined as a short-time break. In this case, the equipment resumes its communication state, for example, based on prestored information on the pre-break operation state of the equipment. It is supposed that power supply to the above-described prior art equipment is cut temporarily because of battery exchange or the like during operation. At this time, information on the operation state of the equipment is prestored in a memory to prevent it from being lost by short breaks. In addition, during normal operation, a capacitor for determining the period of a short break is charged. The capacitor begins to discharge when there is a short break in power supply, and the terminal voltage thereof begins to fall according to a predetermined time constant of a circuit. When the equipment recovers from the short break, the terminal voltage of the capacitor is checked during the activation of the portable telephone equipment. If the voltage is beyond a threshold value, a power failure is determined as a short-time break, and the equipment is caused to resume the pre-break operation state by setting radio part based on prestored information on the pre-break operation state of the equipment.
Owing to the above function, a time-constant circuit corresponding to a set time for allowing resumption is prepared to determine the period of a short break so that the equipment can continue the same operation before and after the break if the period is short.
The term "the set time for allowing resumption" means the time set in the portable telephone equipment to determine whether a power failure is a short break or not for resumption. The time required for a fixed station controlling a portable telephone equipment to determine the abnormal completion of communication is several seconds, but is not always constant. In consequence, unless the above-mentioned set time is set at a time shorter than the decision time, that is, within a time range that the fixed station allows resumption, the equipment will not operate properly, thus exerting an adverse effect on the telephone system.
To effect the resuming function in the prior art portable telephone equipment, whether the period of a short break falls within a resumption allowable time range or not is determined indirectly from the discharge voltage of a capacitor. Therefore, the allowable time range (resumption time) cannot be set precisely because Of the wide margins of error caused by differences in produced capacitors and fluctuations in the time constant of the circuit by temperature. As far as the allowable time range permits, it is desirable that the resumption set time is set as long as possible. In fact, the actual set time may become shorter than desired, and, accordingly, it is difficult to set a long allowable time range (resumption time).